Several of Atom's core packages are maintained in the packages directory of the atom/atom repository. If you would like to use one of these packages as a starting point for your own package, please follow the steps below.
Tip: In most cases, we recommend generating a brand new package or a brand new theme as the starting point for your creation. The guide below applies only to situations where you want to create a package that closely resembles a core Atom package.
For the sake of this guide, let's assume that you want to start with the current code in the one-light-ui package, make some customizations to it, and publish your new package under the name "one-light-ui-plus".
Download the current contents of the atom/atom repository as a zip file
Unzip the file to a temporary location (for example /tmp/atomC:\TEMP\atom)
Copy the contents of the desired package into a working directory for your fork
cp -R /tmp/atom/packages/one-light-ui ~/src/one-light-ui-plusxcopy C:\TEMP\atom\packages\one-light-ui C:\src\one-light-ui-plus /E /H /K
Create a local repository and commit the initial contents
cd ~/src/one-light-ui-plus git init git commit -am "Import core Atom package"
Update the name property in package.json to give your package a unique name
Make the other customizations that you have in mind
Commit your changes
git commit -am "Apply initial customizations"
Create a public repository on github.com for your new package
Follow the instructions in the github.com UI to push your code to your new online repository
Follow the steps in the Publishing guide to publish your new package
The code in the original package will continue to evolve over time, either to fix bugs or to add new enhancements. You may want to incorporate some or all of those updates into your package. To do so, you can follow these steps for merging upstream changes into your package.